Thomas P. O'Brien, who charged a former U.S. Marine with manslaughter and sought to strip a motorcycle gang of its identity, immerses himself in the grand jury investigation of Cardinal Roger Mahony.

LOS ANGELES AND WASHINGTON — In the 16 months since he was sworn in as U.S. attorney in Los Angeles, Thomas P. O'Brien has charged a former U.S. Marine with manslaughter for combat killings in Iraq, tried a Missouri woman whose alleged crime, many would argue, was committed halfway across the country and sought to strip a notorious outlaw motorcycle gang of its very identity.

Now he has immersed himself in a grand jury investigation into Cardinal Roger M. Mahony's handling of sexually predatory priests in the Archdiocese of Los Angeles.

In each of these cases, O'Brien, a former Navy Top Gun instructor and gang prosecutor in the district attorney's office, has either taken on battles that others may have shied away from or employed novel -- some would say shaky -- legal strategies to make his case.

Mahony said in a radio interview Thursday that he was "mystified and puzzled" by news of the probe.

The results of O'Brien's previous prosecutorial pursuits have been mixed. In the case of the ex-Marine, O'Brien's office used a law intended for prosecuting soldiers' spouses or civilian Department of Defense employees living in foreign countries to try Jose Luis Nazario for the killings of four unarmed Iraqi prisoners in Fallouja. Nazario had left the Marines and was no longer a reservist, meaning he was not subject to a court-martial. Rather than simply let him go, O'Brien tried Nazario in civilian court, the first such case in modern history. The case angered some former Marines, who said civilians jurors were not capable of fairly judging decisions made in combat. Nazario was acquitted.

In another case, Lori Drew, the Missouri mother accused of perpetrating an Internet hoax that prompted the suicide of a teenage girl, was charged under a statute more commonly used to go after computer hackers. She was acquitted of the felony charges against her but convicted of three misdemeanors. She is awaiting sentencing, but the judge is considering whether to dismiss the case altogether on grounds that, despite the jury's verdict, prosecutors failed to make their case.

In the Mongols motorcycle gang case, O'Brien's office claimed the rights to the gang's trademarked emblem, empowering officers to seize -- on the spot -- jackets, motorcycles and other items members use to identify themselves.

Rebecca Lonergan, a former federal prosecutor who worked under O'Brien in the U.S. attorney's office and now teaches law at USC, said "there is no doubt that his prosecutions have been very aggressive."

"The question is whether they've been overly aggressive or genuine attempts to make sure that crimes that should get prosecuted do get prosecuted," Lonergan said. "There are two possible motives. One is to do the right thing. The other is less meritorious, which is attention grabbing. I can't say what's going on here."

"Everything we do is in the pursuit of justice," said Thom Mrozek, a spokesman for O'Brien. "We have used innovative legal theories in an effort to vindicate the victims of criminal activity. While this may have generated criticism in some circles, we stand by our decisions."

According to sources familiar with the Mahony investigation, O'Brien is now involved in a grand jury probe aimed at determining whether the cardinal and other church officials committed fraud by failing to inform parishioners of sexually predatory priests in Los Angeles and surrounding areas.

Mahony declined to be interviewed by The Times on Thursday, but told KNX Radio that church officials were cooperating with the investigation and had turned over documents concerning 22 priests.

He said he has acknowledged making mistakes in the handling of abusive priests decades ago and that the church has made significant reforms as a result.

"We admitted . . . all of our failures along the way and so we don't know where this is coming from," he said.

Two years ago, the archdiocese paid $660 million to 508 people who accused priests of sexual abuse. Los Angeles County Dist. Atty. Steve Cooley accused the archdiocese of engaging in a "pattern of obstruction" and investigated church leaders, but never filed charges.

In the current grand jury probe, prosecutors are examining whether Mahony and other church officials could be charged under a federal fraud statute that makes it illegal to "scheme . . . to deprive another of the intangible right of honest services," sources said.

But that may be an uphill battle for prosecutors.

The law against "honest services fraud" has come under increasing attack from legal experts who say it is too broad and open-ended.

Twenty years ago, Congress expanded the anti-fraud statute to combat public corruption and to punish officials who enriched themselves at taxpayers' expense. But it was written so broadly it can be used against private persons, such as a corporate executive who violates his duty of trust.

Still, lawyers who have tracked the law said they were not aware of it ever being used against a church official.